HR: 0800h
AN: H51A-1112    [Abstracts]
TI: Climate-induced variations in the sourcing and weathering of fluvial sediments: Deltaic records from the monsoon-forced Ganges and Brahmaputra dispersal systems
AU: * Youngs, P M
EM: youngs@msrc.sunysb.edu
AF: Stony Brook University, Marine Sciences Research Center, Stony Brook, NY 11794-5000 United States
AU: Goodbred, S L
EM: sgoodbred@notes.cc.sunysb.edu
AF: Stony Brook University, Marine Sciences Research Center, Stony Brook, NY 11794-5000 United States
AU: Kuehl, S A
EM: kuehl@vims.edu
AF: College of William and Mary, Virginia Institute of Marine Science, Gloucester Point, VA 23062 United States
AB: The Ganges-Brahmaputra river system is among the largest in the world and has been shown to respond rapidly to millennial-scale shifts in South Asian Monsoon climate. This response is seen in an order of magnitude shift in both river discharge and sediment deposition from the modern mean throughout the Holocene. The tectonic dynamics of the basin and the high sediment loads have led to thick, high temporal resolution sediment sequences deposited on the delta during this time. Drawing on the unique chemical signatures of the major Himalyan lithologies drained by these rivers, other works have carried out provenance studies on sediments from the Bengal Fan looking for changes over the last glacial-interglacial cycle. These studies show that there has not been significant change in provenance on this time scale, so we examined sediments from the deltaic sequences using the same method of Sr isotope composition. From several long (100m-180m) boreholes from around the delta, we were able to sample specific, separate sub-basins of Ganges and Brahmaputra influence, as well as regions where this influence was mixed. Throughout the Holocene we find no significant change in downcore Sr ratios. However, our results indicate that there is a large departure from the mean $^{87}$Sr/$^{86}$Sr at the base of the Holocene Brahmaputra sequence presumed to date to the last glacial maximum. Initial clay mineralogy and major element data suggest no change in extent of weathering of these sediments, implying a shift in provenance as the cause of this excursion. Early results also indicate unique Sr isotopic compositions between the two rivers, which can be used to delineate specific source areas for each river as well as the delta as a whole. Also noted has been an increase in weathering extent of Ganges deposits over the past 3000 years. Further work on the provenance of these sediments will include studies of the sedimentology, mineralogy, and Nd and Ar isotopic compositions. Clay mineralogy and major element data will continue to be used to reconstruct the weathering history of these sediments as well. With these added data, we expect in the near future to reconstruct the sourcing, weathering, and transport history of the sediments delivered to each drainage basin during this period of significant and rapid climatic change.
DE: 1815 Erosion and sedimentation
DE: 1040 Isotopic composition/chemistry
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting